Oxidation of Thiocyanate to Cyanide Catalyzed by Hemoglobin

Thiocyanate ion is oxidized at acid pH by hydrogen peroxide to sulfate and cyanide. The reaction is catalyzed in the erythrocyte by hemoglobin acting as a peroxidase (donor: H2O2 oxidoreductase, EC 1.11.1.7). Kinetic studies show thiocyanate ion fulfills criteria for a substrate for the peroxidase. The peroxidase activity is enhanced by haptoglobin and inhibited by azide, aminotriazole, fluoride, iodide, and cyanide. No other enzyme catalyzing this reaction was found in hemolysates of bovine blood. Oxyhemoglobin was a more active catalyst than methemoglobin but was rapidly converted to the latter in the incubation system. Equivalent amounts of sulfate and cyanide were produced initially, but the cyanide was converted to cyanate and ammonia. The conversion was not rapid nor catalyzed by hemoglobin except in the presence of thiocyanate. The enzymic reaction resembles the acid-catalyzed oxidation of thiocyanate by hydrogen peroxide. Oxidation of sulfur dicyanide, a proposed intermediate product, was catalyzed also by hemoglobin. Cyanide produced in vivo is converted in part to thiocyanate by sulfur transferase systems. The thiocyanate-cyanide cycle probably accounts for some of the physiological effects of thiocyanate.